Conventionally, for example, an electric power steering apparatus includes an electric motor for imparting a steering assist torque to a rotating operation of a steering wheel, and adjusts the steering assist torque through performance of energization control which changes the current supplied to the electric motor. Such an electric power steering apparatus, which uses an onboard battery as a power source, consumes a large mount of electric power. Therefore, an apparatus proposed in Japanese Patent Application Laid-Open (kokai) No. 2007-91122 includes a secondary power supply apparatus for supplementing the onboard battery. The secondary power supply apparatus is connected in parallel with a power supply line extending from the onboard battery (hereinafter referred to as the “primary power supply apparatus”) to a motor drive circuit. The secondary power supply apparatus is charged by the primary power supply apparatus, and supplies the charged electric power to the motor drive circuit. Further, the proposed apparatus includes a switch for starting and stopping the supply of electricity from the secondary power supply apparatus to the motor drive circuit, and a switch for starting and stopping the charging of electricity from the primary power supply apparatus to the secondary power supply apparatus.